This invention relates to the monitoring of the optical output of a transmitter by means of an optical tap located in the transmission path. It finds particular application in respect of transmitters required to operate with a wide dynamic range of optical output power, such as is liable to occur in the outstations of a TDMA (time division multiple access) system that employs a non-serial form of marshalling. In such a system a plurality of outstations communicate with a basestation on a TDMA basis while the basestation may communicate with its outstations on a broadcast basis. To operate such a system there is a fundamental requirement that the system shall be able to measure, and then equalise, the propagation delays on the different spurs linking the different outstations with the basestation. This measurement can be made by transmitting signals at normal power from each of the outstations to the basestation, and noting the time of receipt of those signals. However, in a practical system the uncertainty in this time occasioned by the different propagation delays means that a substantial period of potential transmission time in the upstream (outstation to basestation) direction needs to be reserved for the receipt of these signals. This method is referred to as `serial marshalling` since it can not be carried out simultaneously with the upstream transmission of data traffic. In the case of non-serial marshalling, an initial marshalling procedure involves the use of transmissions at power levels too low to have significant affect upon normal data traffic, so that these initial marshalling transmissions can therefore be transmitted in parallel with that data traffic. This involves the outstation transmitting a pseudo-random bit sequence which is detected at the basestation by cross-correlation. Such an initial marshalling method is for instance described in U.S. Pat. No. 5,528,596.
The optical source of an outstation is provided by a directly modulated semiconductor laser chip and, because the electro-optic conversion efficiency of such a device varies with temperature and with the effects of ageing, it is conventional practice to regulate its optical power output with the aid of a feedback control loop, deriving a feedback control signal from the photocurrent produced by a monitor photodetector positioned to intercept a part of the laser's emission. An example of such a TDMA transmitter is for example to be found in United Kingdom Patent Application No GB 2 312 346 A.
When initial marshalling commences, the output power of the outstation's laser chip must, under the least favourable conditions, be low enough to avoid producing unacceptably high corruption of data being simultaneously transmitted to the basestation from any other outstation. Typically this transmission will be at a power level too low for the cross-correlation performed at the basestation to detect it. The power level of the transmission must then be incremented in steps until it is large enough to be detected. The size of those steps is regulated on the one hand by the need to make them small enough for a single increment not to raise the power level from undetectable to datacorrupting, and on the other hand to make them large enough to ensure that detection will occur within a reasonably short time interval from the commencement of the initial marshalling.
Implicit in the foregoing is the fact that regulation of the drive applied to the outstation's laser chip in order to regulate the optical power output by that outstation is required, not only during data transmission, but also during the initial marshalling process, and that at least in the initial stages of the initial marshalling process, the optical output power of the outstation needs to be regulated to a power level very much lower than that employed during data transmission, typically commencing at a power level in the region of 40 dB below that of the data bits.
Typically regulation of the drive current applied to a semiconductor diode is achieved by means of a feedback control loop whose photocurrent is taken from a monitor photodetector positioned to receive light emitted from the back-facet of the laser chip. To attempt to use such a back-facet monitor photodetector for regulation that compasses such a dynamic range of drive current introduces the problem that, if the photodetector is sensitive enough to provide a photocurrent of sufficient magnitude for closed loop control of the laser drive current during the initial stages of initial marshalling, the it is very liable to saturate before the laser drive current is at the required level for subsequent data transmission. Alternatively, if the sensitivity of the photodetector is small enough to avoid saturation during data transmission, it is very liable to be too small to provide a photocurrent large enough for closed loop control during the initial stages of initial marshalling.
One way of circumventing the problem of having too low a light level, during at least the initial stages of initial marshalling is to create a lookup table that relates power output to drive current from calibration measurements made on a test jig before the laser is ever brought into service. (Such a test jig would derive its calibration from measurements made with its own photodetector rather than the monitor photodiode of the laser itself). Under these circumstances, due allowance can be made for the effects of temperature, but importantly not for those of ageing.
The absence of a means for making allowance for the effects of ageing gives rise to a magnitude of uncertainty in optical power level output for which due allowance must be made in the devising of the incremental power steps in a manner that avoids the risk of data corruption, and this in turn has the effect of lengthening the time taken to complete initial marshalling.
Reverting attention to general aspects of generating a monitoring signal for use in feedback control, it is typically found desirable to choose a system providing a linear relationship between the magnitude of the monitoring signal and the magnitude of the parameter being monitored.
If such a regime were to be employed for regulating the transmitter output power of an outstation using non-serial marshalling in which the initial marshalling power was required to be 40 dB below that required for the transmission of data, then the corresponding monitor signal during initial marshalling would similarly be 40 dB below that pertaining during the transmission of data. In fact however a back-facet monitor typically provides a response that is somewhat more favourable than a linear response.
This can be seen from measurements performed in respect of a system as depicted in FIG. 1. This system has a laser diode 10 provided with a back-facet monitor photodiode 11. The main output of the laser diode 10 is coupled into one end of a length 12 of single mode optical fibber. The light emerging from the far end of the fibre 12 is optically coupled into an optical power meter 13. In FIG. 2, curve 20 shows how the monitor current provided by the monitor diode 10 varies as a function of power output delivered to meter 13. At low output power levels in the region of 0.5 .mu.W delivered to the power meter 13 by the fibre 12, monitor efficiency is in the region of 1.3 .mu.A/.mu.W, falling off to about 1.0 .mu.A/.mu.W at an output power of about 1 .mu.W, and then falling off further at higher power output levels to an asymptotic value less than 0.45 .mu.A/.mu.W. The reason for this non-linear efficiency characteristic is that the back-facet monitor photodiode can be expected to produce a photocurrent in substantially direct proportion to the total light output (both coherent and non-coherent) from the front facet of the laser diode; that the proportion of incoherent to coherent light emission falls with increasing power output from that forward facet; and that the coherent light is conveyed by the single mode fibre 12 with much less transmission loss than the conveyance of the incoherent light on account of the coherent light being emitted in a solid angle more nearly matched with the acceptance angle of the single mode fibber. If for instance a photocurrent of at least 1 mA were required for closed loop control of the laser drive current control, then it is seen that, as a result of this non-linearity, the feedback loop is operational down to a fibber output power in the region of 0.5 .mu.W. In the absence of the nonlinearity, and assuming the efficiency value of 0.45 .mu.A/.mu.W in respect of coherent light, the corresponding figure is that the feedback loop would be operational only down as far as about 2.2 .mu.W. Actually, an efficiency value of 0.45 .mu.A/.mu.W is impracticably large if the required signal traffic power level is in the region of 2 mW, for this would imply that the monitor photodiode be capable of delivering a photocurrent in excess of about 90 mA without saturating. Thus it is seen that, though the nonlinearity provided by the capture by a back-facet monitor of incoherent light provides a beneficial effect, a still greater non-linearity would be more useful.